1. Field of the Invention
The present invention relates to a method of controlling an electric braking device which is capable of setting a pad clearance without using a position sensor in a motor-driven disc brake, and a device for executing the same.
2. Description of the Related Art
Techniques for setting a pad clearance between the pad and the rotor in a non-braking mode of the braking device are proposed in Japanese Patent Laid-Open Nos. 137841/1997 and 2000-18294.
In the former publication, the control unit has a thrust control function to control the action of the actuator in accordance with a detected signal from the thrust sensor and a position control function for controlling the action of the actuator in accordance with a detected signal from the position sensor. With those functions, when the braking operation is released, a position at which the friction material starts to operate is accurately and reliably corrected to a correct position in accordance with a degree of wear of the friction material.
In the latter publication, the electric braking device includes a differentiating circuit for differentiating an output signal of the thrust sensor, and a circuit for comparing a differentiated value with a threshold value. The clearance control is performed while using a position where the differentiated value of the output signal of the thrust sensor is smaller than the threshold value as the origin.
In the techniques of those publications, the output signals of the position sensor and the thrust sensor are indispensably used in adjusting the pad clearance. Accordingly, it is essential to form the spaces for accommodating the sensors in the caliper. This creates problems such that it is necessary to secure the sensor mounting positions, and the number of wires is increased. The problems will hinder the reduction of the size and weight of the braking device.
Accordingly, an object of the present invention is to provide a control method for an electric braking device which is capable of executing a brake control and a pad clearance control by using only the thrust sensor, and a device for executing the same.
In the present invention, a target value as computed by using a brake pushing force and a predetermined threshold value are compared with each other when a brake switch (3) is in an on state. When a target value as obtained when a brake switch (3) is in an on state is below a predetermined threshold value, the electric motor is controlled so that the target value approaches to and is equal to a predetermined target value (a fixed braking force having such a magnitude as not to cause a dragging of the brake). Thereafter, when the brake switch (3) is turned off, the electric motor (10) is rotated for a predetermined time to retract the pad thrust member, whereby the pad clearance (the return quantity of the pad) is automatically set. Accordingly, the number of sensors used is reduced, and the control is well performed.
To achieve the above object, there is provided a method of controlling an electric braking device which includes a brake switch (3), a pushing force sensor (2), a thrust sensor (15), and an electric motor which moves a pad thrust member in a direction in which the pad thrust member approaches to a rotor (4) which rotates with the vehicle wheel associated therewith, thereby performing a braking operation. In the control method, a brake target value as computed by using an output signal of the pushing force sensor (2) and a threshold value as set to a minute braking force are compared with each other when the brake switch (3) is in an on state. When the brake target value is below the threshold value, the electric motor (10) is controlled with a predetermined value as the target value, and after the brake switch (3) is turned off, the electric motor (10) is rotated for a predetermined time to retract the pad thrust member.
In the control method, the predetermined-time rotation of the electric motor (10) is performed after the brake switch (3) is turned off and a post stage control is continued for a first predetermined time.
Also in the control method, the pad thrust member (17) is coupled to the electric motor through a rotation-linear motion converting mechanism (13) including a gear nut (14) and a screw shaft screw-coupled to the gear nut. The thrust sensor (15) is provided between a bearing rotatably holding the gear nut (14) and a caliper. The motor operated actuator (10) is controlled in accordance with a difference between a brake target value as computed by using an output signal of the pushing force sensor (2) and a braking force obtained from the thrust sensor (15).
According to another aspect of the invention, there is provided an electric braking device having an electric motor which moves a pad thrust member (17) in a direction in which the pad thrust member approaches to a rotor (4) which rotates with the vehicle wheel associated therewith, thereby performing a braking operation. In the electric braking device, the pad thrust member (17) is coupled to the electric motor through a rotation-linear motion converting mechanism (13) including a gear nut (14) and a screw shaft (16) screw-coupled to the gear nut (14). A thrust sensor (15) for measuring a thrust force of the pad thrust member (17) against the rotor (4) is provided between a bearing (21) for rotatably holding the gear nut (14) and a caliper (6).